The present invention relates to apparatus for measuring certain theological characteristics of an oilfield fluid or the like such as a drilling mud or a cement slurry. More particularly, the invention relates to evaluating the tendency of a drilling mud to stick and to the difficulty of eliminating the mud cake.
Drilling a well such as an oilfield, gas, geothermal, or analogous well necessitates the use of various fluids, the rheological characteristics of which must be known as precisely as possible, with measurements preferably being made at the drilling site while the fluid is being prepared. During the drilling operation proper, a fluid or drilling mud is circulated in the drilled hole, which mud acts, inter alia, to cool and lubricate the drilling tool, to lift drilling debris to the surface, to prevent ingress of formation fluid, in particular gas, and to maintain stability by preventing the walls from collapsing. Once the hole has been drilled, casing or coiled tubing is lowered into the hole and is cemented over all or a part of its height, with the cement slurry being injected via the inside of the tubing or casing to then fill the annular space between the tubing and the well wall. In addition to a mechanical function of maintaining the tubing or casing in position, the cementing isolates the various layers of formation traversed by the hole, prevents gas from rising via the annular space and limits the ingress of water into the production well. It also, of course, acts primarily to hold the casing in position.
Very schematically, a cement slurry is constituted by a liquid base (water and soluble additives), and solid materials (cement) in suspension in that base.
The drilling mud is principally constituted by a liquid base which is generally water, oil, or a water-in-oil emulsion type, by salts or other substances in solution in the base, and by solids which are insoluble in the base such as baryte or bentonite which are added to adjust the density and the viscosity of the fluid.
When a fluid is in contact with a relatively porous formation, and if the hydrostatic pressure of the formation is lower than the pressure of the drilling fluid, then the fluid will tend to penetrate into the formation. However, the pores of the formation are generally smaller in size than the solid particles suspended in the fluid. This leads to a filtration phenomenon with the liquid base and the finest solids flowing into the formation and with the less fine solid particles being deposited on the walls to form a cake, known as mud cake when the fluid is a drilling mud.
In itself, mud cake formation is a desirable phenomenon since it tends to render the well walls impermeable and thus limits drilling fluid loss. However, drillpipes, logging tools and casing elements can rub against and become stuck in the cake when such elements stop rotating for a prolonged period, which incidents are difficult to avoid since drilling mud is often circulated for periods which last for several months. Freeing an element stuck in the cake means that a very large torque has to be applied, which torque is not always immediately available and is also associated with a risk of breakage. As a result, drilling can be interrupted, leading to major overcosts.
United Kingdom patent GB-A-2 275 342 describes apparatus for evaluating the tendency of a drilling mud to stick by measuring the torque required to rotate a ball in contact with a porous surface through which a drilling mud is forced in order to form a cake. That apparatus can also measure the quantity of liquid which has flowed through the wall after 30 minutes (fluid loss measurement). However, the disadvantage of that apparatus is that it has been shown that the results are not always reproducible from one experimenter to another and in particular, the torque measured is systematically overvalued by a friction factor which depends on the type of drilling mud and which as a result is not a constant.
The apparatus known from GB-A-2 275 342 is constituted by a pressurised cell having a base which is formed by a grid with its surface facing the inside of the cell being covered with a filter paper in order to simulate the well wall. The cell is provided with a cover which is pierced at its centre by a passage for a rod having the ball fixed to its end. The cell is filled with the mud to be tested and the ball is lowered until it just touches the filter paper. A gas is injected above the mud to simulate the pressure difference between the mud and the formation fluids. As the height of the mud cake increases, the torque T required to initiate rotation of the ball is measured by a sensor associated with the rod. According to the model used by the authors, that torque is proportional to a sticking factor S which is characteristic of a given mud and to txc2xe, where t is the time over which the differential pressure is applied.
French patent application FR-A-2 758 185 describes an improvement to the apparatus described in the patent cited above, the improvement consisting in means, for example bearings, to keep a constant distance which is as small as possible but not zero between the ball and the filter paper, and in driving the ball using a magnetic transmission in order to be able to measure very small torques as are measured in particular with oil-based mud. The improved apparatus of the cited French application has the advantage of being easier to handle, more robust, and thus more suitable for use at a drilling site.
The physical principles on which the measurements of the sticking factor are based using the apparatus known from the two patents cited above are described in detail in IADC/SPE 35100 in an article by Reid, P. I., Meeten, G. H., Way, P. W., Clark, P., Chambers, B. D. and Gilmour entitled xe2x80x9cMechanisms Of Differential Sticking And A Simple Well Site Test For Monitoring And Optimizing Drilling Mud Propertiesxe2x80x9d. The value of the sticking factor is determined by assuming: that the height h of the cake increases in accordance with a function of the type h=xcex2txc2xd, where xcex2xe2x80x94or the growth factorxe2x80x94is constant for a given mud under fixed temperature and pressure conditions; that the growth of the mud cake is not affected by the ball; that there is no sliding at the cake/ball interface; and that the stress exerted by the mud cake is constant over the entire height of the cake (in other words, the cake is homogeneous over its entire height). On these assumptions, the torque T which must be exerted at a time t to cause the ball to rotate can be expressed using a function of the type T=S (txc2xdxe2x88x92t0xc2xd){fraction (3/2)} where t0 is the time between the start of measurements and the time when the mud cake reaches the lowest point of the ball, and S is a factor termed the sticking factor which is characteristic of a drilling mud at a given temperature and pressure.
The critical values for the sticking factor are determined experimentally, which means that new muds presenting high risk can be rejected.
Further, before carrying out the cementing operation, it is often desirable to clean off the mud cake. Depending on the mud used, this well wall cleaning operation will be easier or harder and in particular will depend on the shear rate of the mud. The shear rate forms part of the definition of the sticking factor, but until now, no procedure has been proposed for evaluating it.
The present invention aims to provide new procedures for determining the shear rate of a mud under given temperature and pressure conditions.
The invention provides a method of determining the shear threshold of a drilling mud, the method consisting in placing a detecting head with a spherical surface near to a porous wall, in pouring the drilling mud around the detecting head, in forcing the mud to pass through the porous surface so as to form a mud cake above the porous surface and around the detecting head which has a diameter D, and in measuring both the time required for the mud cake to reach a measurable or pre-determined height, and also the torque opposing rotation of the detecting head corresponding to that height.
In a first variation of the invention, the height of the mud cake is measured at the end of the test by measuring the impression left by the detecting head after forming the cake for a given time t.
In a second variation of the invention, the time required for the cake to reach the bottom of the detecting head is measured from at least two measurements made at two different distances between the porous wall and the detecting head.
In a third variation of the invention, the detecting head is provided with marks such that the torque measured varies substantially at the moment the mud cake reaches the height of one mark.
In all variations of the invention, the procedures proposed also enable the growth factor of the cake to be determined.